support object rest

[Rich-Harris]

This supersedes #1541; I can't figure out how to commit to a pull request.

It doesn't solve the thorny issue we just found in Gitter:

<pre>{JSON.stringify(props)}</pre>

<script>
    export default {
        data: () => ({
            unwanted: 1,
            wanted: 2
        }),

        helpers: {
            JSON
        },

        computed: {
            props: ({ unwanted, ...props }) => props
        }
    };
</script>

After changing the value of 'wanted' a couple of times...

+ expected - actual

-<pre>{"wanted":6,"props":{"wanted":4,"props":{"wanted":2}}}</pre>
+<pre>{"wanted":6}</pre>

[Conduitry]

Test failures look to just be caused by Node 6 not supporting object rest.

Committing to a pull request is something I had to keep re-learning how to do and eventually just made some notes for myself. To wit:

git checkout -b <what you want to call the branch locally>
git pull <https url to fork> <your branch name>
<commit, etc>
git push -u <https url> <branch name>

A little inconvenient, as you need to enter your username and password and can't just use your ssh key, but it does feel tidier than making a new PR.

[Rich-Harris]

Wait, I messed this up. When we have ({ unwanted, ...props }), we're not saying that props is a dependency, we're saying that everything except unwanted is a dependency. (unwanted is also a dependency, since it's there in the object pattern, even though it's only there because we don't want it... rest properties are fun.) That's not expressed in this PR.

[Rich-Harris]

closing in favour of #1628